1. Field of Invention
The present invention relates generally to methods and apparatus for producing particles, and particles formed thereby.
2. Description of Related Art
The enhanced mass-transfer properties and benign nature of supercritical fluid, near-critical fluid and/or compressed gas (hereinafter collectively referred to as “supercritical fluid”), makes it particular suitable for use in the production of particles. One prior art technique, which is often referred to in the art as Particles from Gas-Saturated Solutions (PGSS), employs supercritical fluid for this purpose.
In the conventional PGSS process, supercritical fluid is used to plasticize a material thereby forming a melt. The melt thus formed is then expanded across a pressure drop. As the melt expands, the supercritical fluid changes phase and diffuses out of the melt as a gas, which leads to the formation of particles. A conventional PGSS process is described in U.S. Pat. No. 5,766,636, which is hereby incorporated by reference in its entirety. Advantages of the PGSS process include low processing temperatures for thermally labile compounds, relatively easy scalability and one step processing of particles.
A significant disadvantage of the conventional PGSS process is that it often is not sufficient to lower the viscosity of the melt. This is especially problematic with the processing of many high molecular weight polymers. Because the viscosity of the melt is not sufficiently low and the concentration of the supercritical fluid in the melt is not sufficiently high at feasible operating conditions (i.e., a temperature below about 373 Kelvin (K) and a pressure below about 30 megaPascal (MPa)), efficient particle dispersion and size reduction is difficult.
A particle production technique having benefits of conventional PGSS processing but having improved processability is desirable.